Curved parts and method for manufacturing the same

ABSTRACT

A curved-part forming method for obtaining a curved part by performing forming on a blank formed of a single metal plate, the method including a bending process in which the blank having a curved outline corresponding to a curve of the curved part in a longitudinal direction is bent into a sectional shape corresponding to a division portion of a sectional shape of the curved part; and a joining process in which two or more portions obtained by the bending process are joined together.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2010/067312, with an international filing date of Sep. 28, 2010 (WO 2011/040623 A1, published Apr. 7, 2011), which is based on Japanese Patent Application No. 2009-224515, filed Sep. 29, 2009, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a method of forming plates into curved parts (more specifically, curved frame parts). More particularly, the disclosure relates to a forming method that makes it possible to form high-strength steel sheets having a tensile strength (TS) that is greater than or equal to 590 MPa into curved parts, curved parts, and a method for manufacturing the same.

BACKGROUND

Curved parts have hitherto been obtained by press forming single metal plates. In such press forming, various forming modes including drawing, stretch forming, stretch flanging, and bending are combined. (Press forming will hereunder be referred to as “conventional press forming.”) Further, a method of bending forming a cylindrical material (Japanese Unexamined Patent Application Publication No. 9-30345), a roll forming technology (Japanese Unexamined Patent Application Publication No. 11-129045), and bending forming using a hollow part (Japanese Unexamined Patent Application Publication Nos. 8-174047 and 2005-1490) are proposed. As an example of reinforcing curved parts, a method of filling with resin foam (Japanese Unexamined Patent Application Publication No. 11-348813) is proposed.

Increasing the strength of a steel sheet in accordance with the demand for reducing weight causes at the same time a reduction in drawing ability, stretch forming ability, and stretch flanging ability on the steel sheet. Therefore, in conventional pressing forming, defects such as cracks or wrinkles, occur. In particular, as the shape becomes complex, there are cases where curved parts cannot be obtained. For example, if portions 50A and 50B (which are curved in an X direction and a Y direction in plan view, and in a Z direction) of a curved part 50 shown in FIG. 11 are formed by performing conventional press forming on a single high-strength steel sheet having a tensile strength (TS) that is greater than or equal to 590 MPa, wrinkles occur in a planar section (such as a wrinkle section in FIG. 11), and cracks occur in a vertical wall at a side surface or in flanges (such as a crack section in FIG. 11). It is possible to suppress the occurrence of cracks/wrinkles up to a certain extent by changing the shapes of parts or optimizing forming conditions of, for example, a blank holder. However, in such a method, to satisfy the need of reducing weight, there is a limit with regard to achieving a higher tensile strength (TS) that is greater than 980 MPa.

A method of obtaining high-strength curved parts by performing bending forming or roll forming on cylindrical materials is disclosed in JP '345, JP '045, JP '047 and JP '490. From the viewpoint of formability of the materials and process constraints, it is difficult to obtain complex curved shapes, and there are serious productivity problems such as an increase in the number of processes. For example, when low-strength materials are used, complex shapes can be easily obtained, but parts have insufficient strength. Therefore, there are, for example, technologies for obtaining reinforcing effects by filling with resin foam (JP '813). However, from the viewpoints of costs, production, and recycling, it is actually not easy to say that such technologies are necessarily useful technologies.

That is, in conventional forming methods, when single high-strength steel sheets are used as materials, forming into desired curved parts cannot be performed by one-piece press forming, or, when single low-strength steel sheets are used as materials, forming into curved parts can be performed, but the parts have insufficient strength, thereby making it necessary to, for example, increase the number of reinforcing pats, as a result of which weight is increased.

SUMMARY

We thus provide the following:

-   -   (1) A curved-part forming method for obtaining a curved part by         performing forming on a blank formed of a single metal plate.         The method includes a bending process in which the blank having         a curved outline corresponding to a curve of the curved part in         a longitudinal direction is bent into a sectional shape         corresponding to a division portion of a sectional shape of the         curved part, and a joining process in which two or more portions         obtained by the bending process are joined together.     -   (2) The curved-part forming method according to (1), wherein,         prior to the bending process, a folding line is formed in the         blank, or a cut is further formed in the blank.     -   (3) The curved part manufactured using the curved-part forming         method according to (1) or (2).     -   (4) A curved-part manufacturing method for manufacturing a         curved part using the curved-part forming method according         to (1) or (2).

Since the material is bent and deformed almost without being variously deformed by drawing, stretch forming, and stretch flanging, it is possible to perform one-piece pressing forming of a single high-strength steel sheet into portions of the curved part. In addition, as a result of the shape of the curved part, which is a target shape to be formed, being reflected in the outline of the blank, it is possible to easily obtain parts having high strength and having a complex curved shape that could not be hitherto obtained, enlargement of space due to a reduction in the cross section of the parts, and a large reduction in weight because, for example, plate thickness is reduced and reinforcing parts are not used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example of a curved part.

FIG. 2 is a schematic view of another example of a curved part.

FIG. 3 is a schematic view of yet another example of a curved part.

FIG. 4 is a schematic view of still another example of a curved part.

FIG. 5 is a schematic view of a further example of a curved part.

FIG. 6 is a schematic view of a still further example of a curved part.

FIG. 7 is a schematic view of a selected example of a curved part.

FIG. 8 is a schematic view of another selected example of a curved part.

FIG. 9 is a sectional view of various exemplary sectional shapes of curved parts.

FIG. 10 is a schematic view of examples of how folding lines are formed.

FIG. 11 is a schematic view of an exemplary curved part formed by conventional press forming.

REFERENCE SIGNS

-   1, 2 Blanks -   1F, 2F Flanges, Portions corresponding to flanges -   10, 20 Portions constituting curved parts -   30 Curved part (target to be formed) -   50 Curved part formed by conventional press forming (50A and 50B     denote portions constituting curved part 50)

DETAILED DESCRIPTION

FIGS. 1 to 8 are schematic views of different examples of curved parts.

FIGS. 1 and 2 each show an exemplary case in which a curve of a curved part 30 in a longitudinal direction is along folding lines in only one of two opposite directions. Further, in FIG. 1, the sectional size is constant in the longitudinal direction of the part and, in FIG. 2, the sectional size changes in the longitudinal direction of the part. FIGS. 3 and 4 each show an exemplary case in which a curve of a curved part 30 in the longitudinal direction along folding lines changes from either one of two opposite directions to the other one of the two opposite directions. Further, in FIG. 3, the sectional size is constant in the longitudinal direction of the part and, in FIG. 4, the sectional size changes in the longitudinal direction of the part. FIGS. 5, 6, 7, and 8 each show an exemplary case in which a curve of a curved part 30 in the longitudinal direction is such that the curved part 30 is continuously curved in only one of two opposite directions (FIGS. 7 and 8 each show an exemplary case in which the curved part has a warped sectional shape in the longitudinal direction). Further, in FIG. 5, the sectional size is constant in the longitudinal direction of the part and, in FIGS. 6, 7, and 8, the sectional size changes in the longitudinal direction of the part.

In these examples, two blanks 1 and 2 have the same planar shape, and the planar shape thereof has a side-bend outline corresponding to the curve of the curved part 30, which is a target to be formed, in the longitudinal direction of the curved part 30. The blanks 1 and 2 may be previously provided with working holes or beads and the like. In a bending process, the blanks 1 and 2 are each bent into a sectional shape corresponding to a division portion of a sectional shape of the curved part 30 so that portions 10 and 20 constituting the curved part 30 are formed. Reference numerals 1F and 2F denote portions corresponding to flanges of the blanks 1 and 2 or denote the flanges of the portions 10 and 20. In FIGS. 1 to 8, broken lines and dotted lines that are formed in regions of the shapes of the blanks 1 and 2 represent mountain folding and valley folding, respectively, and indicate places corresponding to bend portions (protrusion edges and recess edges) formed by bending in the bending process. In the bending process, using a die, the blanks are press bended so that forming portions of the blanks become bend portions that are in correspondence with target parts. By press bending, forming materials primarily undergo deformation of bending forming, and are formed into target shapes.

Next, in a joining process, the portions 10 and 20 are joined together to obtain the curved part 30. Joining methods may be any one of, for example, welding, caulking, riveting, and adhesion using an adhesive.

Although the examples shown in FIGS. 1 to 6 are those in which the blanks are formed into a part sectional shape shown in FIG. 9( a), methods and curved parts are not limited thereto. Our methods and curved parts are applicable to cases in which, for example, as shown in FIG. 9( b), the blanks are formed into a part sectional shape that is the reverse of that in FIG. 9( a) at the left and right sides or, as shown in FIG. 9( c), the blanks are formed into a part sectional shape so that the flanges 2F of only the structural portion 20 are bent. The examples shown in FIGS. 7 and 8 are those in which the blanks are formed into a part sectional shape shown in FIG. 9( d).

Although, the examples shown in FIGS. 1 to 6 and FIG. 8 use two blanks having the same planar shape for one curved part, our curved parts are not limited thereto. Our method and curved parts are applicable to cases in which three or more blanks are used for one curved part, with at least one of the blanks having a planar shape that differs from the planar shapes of the remaining blanks.

Further, to increase position precision of the bend portions during the bending, it is desirable to previously provide folding lines in portions of the blanks where the mountain folding and the valley folding are performed. We are not only limited to (continuously) forming the folding lines along an entire bending processing portion. The folding lines may be (intermittently) formed in only portions of the bending processing portion according to the circumstances. As a method of forming the folding lines, it is desirable to use, for example, coining. Another example thereof is a method of continuously transferring the unevenness of a roller surface to surfaces of the materials. Suitable forms of folding lines may be provided by forming V grooves such as that shown in FIG. 10( d), in a linear form (10(a)), a broken-line form (10(b)), or a dotted-line form (10(c)), or in a combination of any of these forms. It is desirable that the depth of the V grooves be less than or equal to 20% of the thickness of a metal plate (abbreviated as “plate thickness”). If the depth of the V grooves exceeds 20% of the plate thickness, the strength of the parts required for, for example, the frame of an automobile may be reduced, or cracks may be formed in the bend portions and, in a high-strength metal material, it is not easy to form the grooves deeply, thereby causing serious production and cost problems.

The shape of the grooves is not limited to a V shape (the grooves are not limited to the V groove shown in FIG. 10( d)) so that the grooves may have various recessed shapes such as U shapes. When the curvature radius of the bend portions is large, a plurality of long and narrow grooves may be formed parallel to each other.

When there are localized portions where wrinkles or cracks are very likely to be formed due to localized excessive stretching or compression during bending (for example, when there are a plurality of localized portions at portions of the blanks corresponding to the flanges that are likely to be subjected to excessive stretch flanging or shrink flanging), previously forming cuts in such localized portions makes it possible to more reliably prevent the formation of cracks and wrinkles, which is desirable.

EXAMPLE 1

Blanks formed of thin steel sheets (material symbols A, B, and C) having plate thicknesses and tensile properties (yield strength YS, tensile strength TS, elongation El) shown in Table 1 were formed into curved parts by forming methods based on Table 2, and the shapes of the obtained curved parts were visually observed, to evaluate the forming methods. The results are as shown in Table 2. In conventional press forming according to a Comparative Example, wrinkles are formed in the wrinkle section and cracks are formed in the crack section shown in FIG. 11, whereas in our Examples, curved parts substantially having target shapes and without having cracks or wrinkles were obtained.

TABLE 1 PLATE MATERIAL THICKNESS YS TS El SYMBOL (mm) (MPa) (MPa) (%) A 1.6 710 990 17 B 1.6 810 1190 13 C 1.6 1300 1500 9

TABLE 2 MATERIAL FORMING No. SYMBOL METHOD RESULT OF FORMING REMARKS 1 A CONVENTIONAL NO CRACKS/WRINKLES COMPARATIVE PRESS FORMING GOOD PRODUCED EXAMPLE 2 A METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 1 WRINKLES PRODUCED 3 A METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 4 WRINKLES PRODUCED 4 A METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 7 WRINKLES PRODUCED 5 B CONVENTIONAL NO CRACKS/WRINKLES COMPARATIVE PRESS FORMING GOOD PRODUCED EXAMPLE 6 B METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 3 WRINKLES PRODUCED 7 B METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 6 WRINKLES PRODUCED 8 B METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 8 WRINKLES PRODUCED 9 C CONVENTIONAL NO CRACKS/WRINKLES COMPARATIVE PRESS FORMING GOOD PRODUCED EXAMPLE 10 C METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 5 WRINKLES PRODUCED 11 C METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 2 WRINKLES PRODUCED 12 C METHOD ILLUS- GOOD NO CRACKS/ EXAMPLE TRATED IN FIG. 7 WRINKLES PRODUCED

EXAMPLE 2

Folding lines provided by V grooves (whose depths are shown in Table 3) in a linear form, a broken-line form, or a dotted-line form such as those shown in FIG. 10, were previously formed in blanks formed of thin steel sheets (material symbols A, B, and C) having plate thicknesses and tensile properties (yield strength YS, tensile strength TS, extension El) shown in Table 1. Then, the blanks were formed into curved parts using forming methods based on Table 3, and the shapes of the obtained curved parts were visually observed, to evaluate the forming methods. The results are as shown in Table 3. In our Examples, cracks or wrinkles were not produced, and curved parts whose shapes more closely matched the target shapes compared to the curved parts in the first Examples (that is, curved parts whose dimensional precisions were good) were obtained.

TABLE 3 V MATERIAL V GROOVE FORMING DIMENSIONAL No. SYMBOL GROOVE DEPTH (%) METHOD RESULT OF FORMING PRECISION REMARKS 1 A LINEAR 7 METHOD GOOD NO CRACKS/ GOOD EXAMPLE FORM ILLUSTRATED WRINKLES IN FIG. 1 PRODUCED 2 A LINEAR 6 METHOD GOOD NO CRACKS/ GOOD EXAMPLE FORM ILLUSTRATED WRINKLES IN FIG. 2 PRODUCED 3 A BROKEN- 12 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 3 PRODUCED 4 A BROKEN- 19 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 4 PRODUCED 5 A DOTTED- 10 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 5 PRODUCED 6 A DOTTED- 16 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 6 PRODUCED 7 A LINEAR 12 METHOD GOOD NO CRACKS/ GOOD EXAMPLE FORM ILLUSTRATED WRINKLES IN FIG. 7 PRODUCED 8 A LINEAR 5 METHOD GOOD NO CRACKS/ GOOD EXAMPLE FORM ILLUSTRATED WRINKLES IN FIG. 8 PRODUCED 9 B LINEAR 10 METHOD GOOD NO CRACKS/ GOOD EXAMPLE FORM ILLUSTRATED WRINKLES IN FIG. 5 PRODUCED 10 B LINEAR 8 METHOD GOOD NO CRACKS/ GOOD EXAMPLE FORM ILLUSTRATED WRINKLES IN FIG. 6 PRODUCED 11 B DOTTED- 4 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 1 PRODUCED 12 B DOTTED- 15 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 2 PRODUCED 13 B BROKEN- 6 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 3 PRODUCED 14 B BROKEN- 13 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 4 PRODUCED 15 B DOTTED- 16 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 7 PRODUCED 16 B DOTTED- 6 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 8 PRODUCED 17 C BROKEN- 8 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 3 PRODUCED 18 C BROKEN- 12 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 4 PRODUCED 19 C DOTTED- 4 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 5 PRODUCED 20 C DOTTED- 9 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 6 PRODUCED 21 C LINEAR 3 METHOD GOOD NO CRACKS/ GOOD EXAMPLE FORM ILLUSTRATED WRINKLES IN FIG. 1 PRODUCED 22 C LINEAR 5 METHOD GOOD NO CRACKS/ GOOD EXAMPLE FORM ILLUSTRATED WRINKLES IN FIG. 2 PRODUCED 23 C BROKEN- 5 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 7 PRODUCED 24 C BROKEN- 10 METHOD GOOD NO CRACKS/ GOOD EXAMPLE LINE ILLUSTRATED WRINKLES FORM IN FIG. 8 PRODUCED 

1. A curved-part forming method for obtaining a curved part by performing forming on a blank formed of a single metal plate, the method comprising: a bending process in which the blank having a curved outline corresponding to a curve of the curved part in a longitudinal direction is bent into a sectional shape corresponding to a division portion of a sectional shape of the curved part; and a joining process in which two or more portions obtained by the bending process are joined together.
 2. The method according to claim 1, wherein, prior to the bending process, a folding line is formed in the blank or a cut is further formed in the blank.
 3. A curved part manufactured according to the method of claim
 1. 4. A curved part manufactured according to the method of claim
 2. 